The present disclosure relates generally to a rotary-wing aircraft and, more particularly, to a rotary wing transmission gearbox system which provides variable speeds to facilitate high speed and low speed flight profiles.
The forward airspeed of a conventional rotary wing aircraft is limited by a number of factors. Among these is the tendency of the retreating blade to stall at high forward airspeeds. As the forward airspeed increases, the airflow velocity across the retreating blade slows such that the blade may approach a stall condition. In contrast, the airflow velocity across the advancing blade increases with increasing forward speed. Conventional rotors must be operated at airspeeds lower than those which cause reverse airflow across a substantial part of the retreating blade and at an rpm low enough to alleviate any potential compressibility Mach number problems at the tip of the advancing blade. This has effectively limited forward airspeeds of conventional helicopters to approximately 180 knots.
A rotary wing aircraft with a coaxial counter-rotating rigid rotor system is capable of higher speeds as compared to conventional single rotor helicopters partly due to the balance of lift between the advancing sides of the main rotor blades on the upper and lower rotor systems. In addition, the retreating side of the rotor discs are also generally free from classic retreating blade stall due to offloading of the retreating disc sector with increasing airspeed to obtain roll equilibrium by balancing the net effects of the equal and opposite moments produced by the advancing sectors of the upper and lower counter-rotating rotor systems. To still further increase airspeed, a compound rotary wing aircraft may incorporate supplemental translational thrust.
For rotary wing aircraft with a coaxial counter rotating rotor system, it is usually unnecessary to dedicate a tail rotor for balancing the torque generated by the main rotors. This allows rotary wing aircraft with coaxial counter rotating rotors to include a tail drive system for propulsion purposes. Currently available tail drive systems are typically powered by operatively connecting the drive system to one or more engines or by operatively connecting the drive system to run parallel to a main gearbox that drives the main rotor or rotors. Such configurations typically do not provide the ability to lower the RPM of the tail drive system without affecting the RPM of the main rotor system, and vice versa. The industry is always receptive to improvements in tail drive systems, particularly those that offer greater control over rotor speed.